Translating OpeGLES1.1 fixed function pipeline to programmable pipeline on the fly

Question

Is it possible to emulate the completed fixed function pipeline with shaders on the fly? By on the fly mean not rewriting the fixed function code to use shaders but sort of an intermediate driver which receives fixed function GLES calls (possibly caching it for full one frame as there is no direct one to one translation from fixed to programmable pipeline) and outputs equivalent GLES2.0 calls.

And even if it possible then how much work would it really be?

Answer 1

For most of ES 1.1, that looks pretty straightforward. All the typical fixed functionality like transformations, lights, and materials, translates directly into shader code.

For a complete replacement, you would obviously have to implement all the functionality. From skimming over the ES 1.1 entry points, I spotted a few items that would not directly translate to ES 2.0, where the last of these looks particularly problematic:

• Arbitrary clipping planes. This is not available in ES 2.0, but not terribly hard to emulate in shaders by calculating a distance in the vertex shader, and then discarding the clipped fragments in the fragment shader.
• ES 1.1 has something called "palette textures". From my understanding, it looks somewhat painful to implement in ES 2.0, but possible. You would probably need two textures, one for the indices, and one for the palette, with two levels of sampling in the fragment shader.
• ES 1.1 supports logical operations (glLogicOp) as part of the per-fragment operations that are executed after the fragment shader. ES 2.0 does not have this, and I can't think of a good way to replicate it. The only thing that comes to mind is to render, read back the result, do the logical operation on the CPU, and then render the resulting image. And you would have to do that every time the operation is changed.